Genome-based bioinformatics and advances in molecular sciences have considerably enriched our knowledge in understanding of the molecular mechanisms of carcinogenesis and cancer metastasis, which in turn significantly impact strategies in anticancer drug research and development. For instance, much effort has been made on developing anticancer drugs targeted at a particular cellular signal transduction pathway (one drug, one target). Consequently, several novel inhibitors have been successfully introduced into clinical practice, including inhibitors of human epidermal growth factor receptors (EGFRis).
Human EGFRs are attractive targets of cancer therapeutics owing to their abnormal expression profile in many epithelial tumors and their influence on the growth and survival of cancer cells. EGFRs are 170-180 kD transmembrane glycoproteins of the ErbB family consisting of four receptors: EGFR (HER1 or ErbB1); HER2 (neu or ErbB2); HER3 (ErbB3); and HER4 (ErbB4) (Herbst and Shin 2002; Hynes and Lane 2005). All EGFRs have an extracellular ligand-binding domain, a single membrane-spanning domain and a cytoplasmic tyrosine-kinase-containing domain (Hynes and Lane 2005). So far 10 EGFR ligands have been reported (Higashiyama, Abraham et al. 1991; Kataoka 2009). These EGFR ligands can be classified as three groups on the basis of their binding specificity to the receptors: the first consists of EGF, TGF-α and amphiregulin (AR) that are capable of binding specifically to EGFR; and the second includes β-cellulin, heparin-binding EGF (HB-EGF) and epiregulin, which show dual specificity, binding both EGFR and ErbB4. The third group is composed of the neuregulins (NRGs) and forms two subgroups on the basis of their capacity to bind ErbB3 and ErbB4 (NRG1 and NRG2) or only ErbB4 (NRG3 and NRG4) (Hynes and Lane 2005). It is of importance to note that no EGFR ligands bind to HER2 (ErbB2). However, HER2 can form heterodimers with all other members (EGFR, HER3 or HER4), and have a greater capacity for transduction of cell growth signals than homodimers, and act synergistically to promote cellular transformation (Graus-Porta, Beerli et al. 1997; Kataoka 2009). Moreover, HER2 acts independently as a major pathogenic factor in malignancy, in particular breast cancer (Hynes and MacDonald 2009).
EGFRs and their ligands are highly expressed in epithelial tumors, such as head- and neck squamous cell carcinomas, colorectal cancer, and non-small-cell lung cancer, and are important regulators of cancer cell growth, angiogenesis and metastasis. Activation of EGFRs initiates downstream signaling cascades, including those involving PI3K/Akt, Ras/Raf/mitogen-activated protein kinase (MAPK), and STAT3, thereby trigging a variety of cellular response associated with the promotion of tumor growth, proliferation, survival, angiogenesis, invasion, and metastasis (Herbst and Shin 2002; Solomon, Hagekyriakou et al. 2003; Yu and Jove 2004; Hynes and Lane 2005; Takeuchi and Ito 2010). Thus, EGFRs have become important targets for anticancer drugs. Three generations of EGFRi have been developed, and many of them have been marketed to treat local advanced or metastatic non-small cell lung cancer, recurrent or metastatic squamous cell carcinoma, or EGFR-expressing metastatic colorectal cancer. Among them Erltotinb (Tarceva) and Gefitinib (Iressa) are synthetic small molecules, whereas Cetuximab and Vectibix are humanoid monoclonal antibodies. Consequently, R&D in exploring new idea EGFRs inhibitors has been intensified (Hynes and Lane 2005; Christoffersen, Guren et al. 2009; Kataoka 2009). Most recently, FDA approved AZD9291, a third generation of EGFRi developed by AstraZeneca, that targets non-small cell lung cancer patients whose tumors share a T790M EGFR mutation (for reference, see: http://www.fiercebiotech.com/story/astrazeneca-wins-big-fast-fda-ok-az9291-lung-cancer/2015-11-13).
Like many other single-molecule-targeted chemotherapeutic drugs, EGFR inhibitors (EGFRis) are expected to have the problem of acquired resistance (Mencher and Wang 2005). Although tumors containing activating EGFR mutations (EGFRms; e.g., deletion in exon 19 or a L858R point mutation) initially respond very well to EGFRi treatment, almost all tumors will develop acquired resistance to these tyrosine kinase inhibitors (TKIs) within 9 to 15 months (Mok, Wu et al. 2009; Rosell, Carcereny et al. 2012). Various mechanisms have been identified to be involved in EGFRi acquired resistance, including acquisition of a second mutation in EGFR, e.g., T790M (a substitution of threonine at the “gatekeeper” amino acid 790 to methionine), HER2 amplification, MET Amplification, PIK3CA mutation, NF1 loss, NF1 loss, and TM4SF5-mediated epithelial-mesenchymal transition (EMT) (Kobayashi, Boggon et al. 2005; Bean, Brennan et al. 2007; Engelman, Zejnullahu et al. 2007; Lee, Kim et al. 2012; Takezawa, Pirazzoli et al. 2012; Ohashi, Sequist et al. 2012; de Bruin, Cowell et al. 2014). Among them, T790M is the most common resistance mechanism—it is detected in more than 50% tumor cells from acquired refractory patients (Kobayashi, Boggon et al. 2005). The T790M mutation is believed to render the receptor refractory to inhibition by these reversible EGFRis through exerting effects on both steric hindrance and increased ATP affinity (Yun, Mengwasser et al. 2008; Sos, Rode et al. 2010).
To overcome acquired resistance in the first generation of EGFRis, the second and third generations of EGFRis are irreversible inhibitors of EGFR and specifically target T790M. As monotherapy, the second generation of EGFRis failed to overcome T790M-mediated resistance clinically (Eskens, Mom et al. 2008; Miller, Hirsh et al. 2012; Katakami, Atagi et al. 2013), and the third generation EGFRis, such as AZD9291, acquired a new mechanism of resistance. Eberlein et al. recently reported the heterogeneous mechanisms of resistance within populations of EGFR-mutant cells (PC9 and/or NCI-H1975) to current and newly developed EGFR tyrosine kinase inhibitors, including AZD9291 (Eberlein, Stetson et al. 2015).
In addition, earlier studies showed that combination of EGFRi with standard conventional chemotherapeutic agents, such as platinum drugs did not achieve overall survival (OS) benefits (Herbst, Prager et al. 2005; Gatzemeier, Pluzanska et al. 2007).
Thus, there is a need to improve the ability of EGFRi to inhibit tumor cell growth.